chromium: Refactor updater entirely in Nix

The update.sh shell script now is only a call to nix-build, which does
all the hard work of updating the Chromium source channels and the
plugins. It results in a store path with the new sources.nix that
replaces the already existing sources.nix.

Along the way, this has led to a quite massive workaround, which abuses
MD5 collisions to detect whether an URL is existing, because something
like builtins.tryEval (builtins.fetchurl url) unfortunately doesn't
work. Further explanations and implementation details are documented in
the actual implementation.

The drawback of this is that we don't have nice status messages anymore,
but on the upside we have a more robust generation of the sources.nix
file, which now also should work properly on missing upstream
sources/binaries.

This also makes it much easier to implement fetching non-GNU/Linux
versions of Chromium and we have all values from omahaproxy available as
an attribute set (see the csv2nix and channels attributes in the update
attribute).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>